I am currently making more homebrew and one of my class features enables the character to transform their arm into a weapon, the bonus with that being that as long as the weapon type doesn't have the heavy property it counts as a finesse weapon for the character. 
I tried doing this with the modifier Weapon Property - Finesse but it doesn't appear to have worked, my other idea would be to add a modifier that is a bonus to strength attacks but I don't think you can do {{modifier:dex}} in the fixed value field. 

You cannot. You can only add actual weapon properties to actual weapons.

No, you cannot add snippets as fixed values because they are not fixed values. “Modifier:Dex” would be the opposite of a fixed value. A “fixed value” is “fixed” because it doesn’t change at all. (And finesse doesn’t add any bonuses, it allows them to use Dex instead, not in addition to.)

You would have to just include everything in as plain text and the player would have to add the actual weapons to their character sheet and adjust the attack and damage rolls according directly on their sheet.